1. Field of the Invention
The present invention relates to an improvement in an electrical discharge machine, and particularly to an electrical discharge machine which prevents a workpiece from oxidizing and rusting during machining setup or a machining operation, thereby contributing to machined surface quality.
2. Description of the Background Art
In known electrical discharge machining using an aqueous dielectric fluid, oxygen dissolved in the dielectric fluid may affect a workpiece, generating an oxidized layer, rust, etc., in the surface of the workpiece. It is known that among workpieces made of ferrous (Fe) materials, those of S55C, NAC and other materials are especially easily rusted. Also, raw materials are more easily rusted than hardened ones. It is also known that among workpieces made of non-ferrous materials, titanium (Ti) and others oxidize at the surface, changing in color. In addition to the influence on the materials as described above, oxidization and/or rusting involves the following various factors:
(1) Oxidization and/or rusting is apt to progress when the dielectric fluid is not flowing, or is in a rest state, as compared to when it is flowing.
(2) Oxidization and/or rusting is more apt to progress as the electrical conductance of the dielectric fluid becomes lower.
(3) Oxidization and/or rusting is hastened when a potential difference is impressed from outside so that the workpiece becomes a positive pole (positive pole oxidization).
To prevent the workpiece surface from rusting, as disclosed in, for example, Japanese Patent Publication No. 137524 of 1983, the negative pole of an auxiliary power supply provided separately from an electrical discharge machining power supply is connected to a workpiece, with the positive pole thereof (in contact with a dielectric fluid) connected to the workpiece via the dielectric fluid and a workpiece mounting table, whereby the workpiece becomes a negative pole and rust is prevented.
An electrical discharge machine known in the art will now be described with reference to FIG. 10, which illustrates the arrangement of a commonly known wirecut electrical discharge machine. The numeral 1 indicates a wire electrode, 2 a workpiece, 3 a wire bobbin, 4a and 4b upper and lower dielectric fluid nozzles, respectively, and 5 an electrical feeder for feeding the wire electrode 1 with electricity, 6 tension rollers for providing the wire electrode 1 with tension, 8 a machining power supply for supplying a machining current to a machining gap formed between the wire electrode 1 and the workpiece 2, 13 an auxiliary power supply, 14 a secondary electrode (float electrode), 15 a machining tank, and 16 a surface plate for securing the workpiece 2. Auxiliary power supply 13 is a battery power supply which is lower in voltage, e.g., approximately 9V, than the machining power supply 8, and the negative pole thereof is connected to the workpiece 2 and the positive pole thereof to the secondary electrode 14 floating on the surface of a dielectric fluid.
The operation of said machine according to the prior art will now be described. Referring to FIG. 10, the wire electrode 1 runs under tension which is imparted by the tension rollers 6, and the machining current is supplied to the wire electrode 1 by the machining power supply 8 through the electrical feeder 5. The machining gap formed by the workpiece 2 and the wire electrode 1 is supplied with pure water, which acts as the dielectric fluid, or an aqueous dielectric fluid, which includes silicon or polymer compounds, etc., as additives, through the dielectric fluid nozzles 4a, 4b from the top and bottom. A discharge is generated across the machining gap, thereby machining the workpiece 2. During non-machining intervals, a microcurrent flows from the auxiliary power supply 13 to the secondary electrode 14, the dielectric fluid, the surface plate 16 and the workpiece 2 in this sequence. As described above, when the electrical discharge machining comes to a stop, the microcurrent supplied by the auxiliary power supply 13 causes the workpiece 2 to be a negative pole and stops the oxidization of the workpiece 2, preventing rusting.
The electrical discharge machine of the prior art arranged as described above prevents the workpiece from oxidizing during non-machining times such as setup, preparation and post-machining periods but cannot prevent the workpiece from oxidizing during machining. Also, when the electrical conductance of the dielectric fluid has become high before, during and after the machining time, the secondary electrode is eroded by electrolytic action and deposits may form on the workpiece surface. Conversely, when the electrical conductance of the dielectric fluid has become low, the current flowing between the secondary electrode and the workpiece decreases, reducing the effect of preventing the workpiece from oxidizing and rusting. Particularly when the electrical conductance has increased during the machining time, the deposition of the secondary electrode material on the workpiece adversely affects machining velocity and machining accuracy. To prevent this, the current flowing between the workpiece and the secondary electrode must be maintained at an optimum value.